Dysmorphic sacra
In the early 1990s, the percutaneous fixation via SI screws began to be used to stabilize posterior pelvic ring, sacral fractures and sacroiliac joint injuries. Compared with the classical method, this new method decreases operating time, soft tissue injury, blood loss, and infection risk [15]. However, the incidence of screw malposition is 3%-25% [16, 17], and the rate of neurological damage has approached to 18% [2, 18, 19], as a result of complex pelvic osteology and especially sacral dysmorphism.
In our study, the “residual disc” could be seen on CT scans for almost samples (98.1%). Weigelt found that the “residual disc” was the most frequent detected sign which present in 70% of all patients [12]. Miller had reported that the “residual disc” could show in persons with non-dysmorphic sacral anatomy[10]. Above all, the “residual disc” was not regarded as a dysmorphic characteristic in our study. In 1996, Routt et al. identified six characteristics of upper sacral dysmorphism in 35% of cases (males: 30%, females: 41%) by x-ray [1]. Compared with the results of Routt, although excluded “residual disc”, the 39.3% dysmorphism rate (males: 34.6%, females: 45.9%) in our study was still higher. Wu studied another four categories of dysmorphic sacra: accessory auricular surface, sacral skewness, transitional vertebra and sacral spina bifida occulta, and the overall rate of variations was 58.1% (males: 57.4%, females: 59.5%) [20]. Hasenboehler et al. only found 14.5% dysmorphic sacra (males: 12.2%, females: 19.2%), which include four dysmorphic types: increased alar slope, obliquity of the residual transverse process on the sacral ala, anomaly of the first sacral anterior neural foramina, and sacralized L5 or lumbarized S1 vertebrae [9]. In our opinion, the classification of Routt was more conducive to guide the safe placement of the upper SI screws in clinical.
Although, in the classification of Routt, there are six dysmorphic characteristics, only “sacrum not recessed” and “acute alar slope” complicate the safe placement of SI screws much. The alar slope is more acute in these patients, and it is associated with a notable inferior-lateral-posterior to superior-middle-anterior orientation. This oblique dysmorphic alar osteology makes TISI screw fixation impossible. Both the two characteristics often show on one sacrum. This phenomenon may be conducted by: in the growth and development of our body, the middle part of the sacral ala develops with the rising sacral body, however the lateral part is still connected with the inferior sacroiliac joint. As the rising of the upper sacral body, the round foramen may be stretched non-circular. The mammillary processes are deformed or under developed residual transverse processes from the sacralized L5 vertebral body. Above all, the appearance of the main dysmorphism often combines with minor dysmorphism.
Since the lumbosacral junction is one of the most variable regions of the spinal column[21], Miller thought when the sacrum fuses excessively cranially, creating sacralized L5 [10]. Therefore, the “sacrum not recessed” might be the representation of the “transitional vertebra”. Due to the different conditions of patients in our study, it is difficult to get the whole spin CT scans to judge whether the upper sacrum is the transitional vertebra.
Indications of upper SI screw fixation for dysmorphic sacrum
For non-dysmorphic sacrum, indications for upper SI screw fixation include complete sacral fractures, sacroiliac joint disruptions, and combinations of these posterior pelvic injuries followed reduction. Incomplete sacral fractures and sacroiliac joint disruptions that contribute to pelvic ring instability may be addressed with SI screw fixation [10].
In 1988, Denis [22] classified the sacrum into three zones: the region of the alar (zone Ⅰ), the sacral foramina (zone Ⅱ), and the central sacral canal (zone Ⅲ). As we all know that, only when all the screw thread pass the fracture line, the fracture fragments could be rigidly fixed. The thread length of the 7.3 mm pull screw was generally 16 mm or 32 mm. In our study, the length of the screw in Denis Ⅲ zone was shorter than 32 mm for all subjects and shorter than 16 mm for 70% of patients. The lengths in Denis Ⅱ+Ⅲ zones for 90% of patients were longer than 32 mm and 100% exceed 16 mm. Therefore, the fractures in the Denis Ⅲ zone could not be rigidly fixed by the upper SI screw, however, they could be treated by classical methods such as posterior planting and sacral bar. On the other hand, as reported [3, 23], patients with a dysmorphic sacrum typically have a safe zone at the second sacral segment that can accept a TISI screw. For these patients, the second sacral segment almost always provides a larger safe zone for screw insertion than does the upper sacral segment.
Screw fixation technique for dysmorphic sacrum
Before the operation, good-quality plain pelvic radiographs and CT scans could be used to help the doctor to identify the dysmorphic sacrum and to estimate the orientation of the upper vertebral pedicle. The inclination of upper SI screw oriented from posterior to anterior was about 20°, and from caudal to cranial was about 30°. As the existence of anterior and cranial inclination angles, the surgeon must know that the insertion point on the skin was more caudal and more posterior than non-dysmorphic sacra. The mean rates of LP-PSIS/LAIIS-PSIS were 0.36 for males and 0.32 for females. The mean values of LPM were 8.81 mm for males and − 4.13 mm for females. Therefore, we infer that the insertion point on the bone surface was nearly located at the rear third of the connecting line from AIIS to PSIS. During the inserting procedure, in the fluoroscopic inlet view, the screw was located in the sacral alar and just posterior to the upper nerve tunnel. In the fluoroscopic outlet view, the screw was located in the sacral alar and just superior to the upper nerve tunnel [10]. What needs special attention was that, in the true lateral sacral view, the tip of the screw lies just cranial-anterior to the iliac cortical density (ICD) line for the main dysmorphic sacra. However, in the non-dysmorphic and minor dysmorphic sacra, the ICD line is coplanar with the anterior sacral alar cortical bone, so the tip of the screw must lies caudal-posterior to the ICD line[10]. Koning had reported that 37% of men and 34% of women don not have a complete osseous corridor to safely pass an 8 mm TISI screw across the S1 vertebral body [24]. In Mendel’s study, 20% of the 125 pelvises did not show a sufficient S1 corridor for a 7.3 mm screw [23]. In our study, the min diameters of the corridor were (16.31 ± 2.40) mm for males and (15.07 ± 1.58) mm for females. The full length of screw were (80.13 ± 5.14) mm for males and (80.13 ± 5.14) mm for females. Therefore, no more than two 7.3*80 mm SI screws were recommended to be placed in the upper sacral segment.